This invention relates to optical filters for producing efficient bandpass reflection and transmission with very narrower line width and low sidebands. More particularly, this invention is related to the means for reflectance and transmission of the radiation (light) of comparatively longer wavelength.
Optical Interference films are used in most optical systems to control or enhance spectral performance. The examples of classical applications include (de) multiplexing, guiding, deflecting and modulation of optical beams for the systems such as holography, integrated optics, optical communications, acousto-optics, nonlinear optics, and spectroscopy. These interference films are thin layers or blends of optical materials of difference refractive index. When light passes through a change in refractive index, partial reflection occurs. The coherence of these subtle reflections determines the nature of the filter's optical spectrum. Conventionally, there are two ways to fabricate the interference filters; (i) Discrete stacks (nλ/4 thickness, where λ and n optical wavelength and integer) and (ii) Rugate. Discrete stack filters are alternating layers of optical materials. Rugate filters on the other hand are a continuous graded, periodic blend of two optical materials.
In the case of discrete stacks light reflected within the high index layers will not suffer any phase shift on reflection while that reflected from a low index layer will suffers a phase shift of 180 degrees. Thus the various component of the incident light produced by successive reflections though out the assembly will appear at the front surface all in phases so that they will combine constructively. This implies that the effective reflectance of the assemble can be made very high indeed, as high may be desired, merely by increasing the number of layers. This is the basis form of high reflectance films. When such coating films are constructed it is found that the reflectance remains high over a limited range of wavelengths, depending on the ratio of high and low refractive indices. Outside this region reflectance changes abruptly to zero. Because of this behavior, the quarter wave stack is frequently used as the basic building block for many types of thin film filter. On the other hands, in the case of Rugate filters, the continuousness of the material refractive index is achieved by mixing ratio of the material blend. This determines the intermediate refractive index of the film. Co-deposition makes Rugate filters extremely challenging to fabricate than discrete stacks.
These two design methodologies provide a technology to fabricate thin film based reflection filters, especially in near infrared regions. When designing the comparatively longer infrared specially mid-infrared or long-infrared ranging from 3 μm to 12 μm, there is a serious limitation. For example, if the filter is optimized for 8 μm then the quarter wave plate thickness comes out to be thick for the IR materials, for examples around 0.83 μm for Zinc Selenide (ZnSe) and 0.89 μm for Zinc Sulphide (ZnS). FIG. 1 shows the simulated results of the filter using sixty pairs of quarter wavelength stacks of ZnSe/ZnS. For 30 pairs of such stacks, we obtain a total thickness around 51 μm and 100 μm for 60 pairs. Total thickness of deposited materials more than few microns give rise to defects like cracks due to excessive stress creation, and thereby, make unstable in fabrication. Besides, under different temperature operation, the filter made from this technique (quarter wavelength stacks) showed unstable due to the crack formation due to mismatching of the thermal coefficient of expansion (TCE). Besides, this unstability in fabrication and operation, the wavelength band (at full wave at half maximum) achieved is also broader. Side lobes are seen at the side of the peak if the quarter-wavelength stacks are used. These could be minimized in some extend by using of Rugate filter design. However we cannot still achieve much narrower bandwidth even using of these two techniques.
It is highly desirable to have a different technique by which the filter can be deigned with lower thickness of the layers and the filter can provide filter specific wavelength having the narrower bandwidth.
According to the current invention, narrow band, high reflectivity optical elements in the mid to long infrared wavelengths for astronomical, commercial, and industrial application can be designed which could be operated under different temperature. The manufacturing thereof is also simpler as compared with the prior art. Some applications include chemical agent detection, atmospheric environment sensing, and laser surgery.